1. Field of the Invention
The present invention relates to video recorders for use with a magnetic tape or similar recording medium for intermittently recording thereon video signals obtained from a monitoring TV camera installed, for example, at the entrance of a bank, commodity store or house.
2. Description of the Related Art
Generally with video recorders of the helical scan type, the signal bearing surface of a magnetic tape running in one direction is traced in an oblique direction with a magnetic head to record a series of video signals on a plurality of tracks thereby formed on the signal bearing surface. Since the video recorder connected to a monitoring TV camera records video signals on a magnetic tape of given length over a long period of time, the video signals obtained by the camera are recorded "intermittently," i.e., as thinned out in a predetermined cycle, on the magnetic tape.
For intermittent recording, digital video recorders can be used which are adapted to digitize the analog video signals obtained by a TV camera and record the digital signals on a magnetic tape.
FIG. 6 shows a processing example wherein digital video signals which are sequential in time are intermittently recorded on a magnetic tape to obtain a recording time twice the real time. In the case where items of time series data 1, 2, 3, 4, . . . are input which are obtained by dividing a series of digital video signals into frames or groups each comprising a plurality of frames as shown in FIG. 6(a) , these items of input data are recorded on the magnetic tape, with every other data unit thinned out with a cycle T as shown in FIG. 6(c)
In this case, the magnetic tape repeats running and stopping alternating in a cycle of T/2 as shown in FIG. 6(b) to record images during running.
In this intermittent recording operation, however, the running and stopping of the magnetic tape alternate in repetition with a cycle shorter than the cycle T of intermittent recording, so that the tape fails to travel with stability every time. Consequently, the head traces the tape at an angle different from the head trace angle of the normal recording operation.
Accordingly, playback of the magnetic tape used for intermittent recording requires a complex procedure such as altering the head trace angle in conformity with the head trace angle used for recording. Especially in the case of digital video recorders, normal reproduction of images requires accurate reproduction of data permitting correction of errors, whereas if head tracing involves an angular deviation in playing back the magnetic tape used for intermittent recording, there arises a problem in that the recorded data can not be read from the tape accurately, failing to afford a normal reproduction.